Cerebrospinal fluid concentrations of homovanillic acid, 5-hydroxyindoleacetic acid, norepinephrine and 3-methoxy 4-hydroxyphenylethylene glycol did not differ significantly between patients with Alzheimer's disease and age matched controls, and were not correlated with age in healthy men. Spinal fluid concentrations of choline increased with age in healthy men, and were higher in young adults with Down syndrome than in age matched controls. CSF biopterin, a coenzyme for tyrosine and tryptophan hydroxylase, was lower in patients with Alzheimer's disease that in controls, and was correlated with concentrations of 5-hydroxy- indoleacetic acid and homovanillic acid. Corticotropin releasing factor, a neuropeptide, was significantly reduced in the cerebrospinal fluid of patients with Alzheimer's disease as compared to controls, as was peptidyl-alpha=amidation activity, suggesting a loss of neurons which produce amidated neuropeptides. Rations of albumin and immunoglobulin between cerebrospinal fluid and plasma were normal in Alzheimer's patients, suggesting that the blood brain barrier is intact.